This invention relates to optical systems in general, and more particularly to an improved arrangement in an optical system which permits viewing an image projected on a wafer or the like without the drawbacks of viewing through the back of the master.
In a copending application of H. S. Hemstreet et al., Ser. No. 339,860, filed Mar. 9, 1973 and assigned to the same assignee as the present invention, an optical projection and scanning apparatus for the manufacture of integrated circuits is disclosed. In the disclosed projection apparatus alignment between a mask or master and a wafer is required prior to scanning the mask to expose the wafer. Viewing systems for accomplishing such alignments are disclosed in that application and various specialized features and aspects thereof are also described in an application of D. A. Markle et al. entitled "Coarse Viewing System", Ser. No. 509,597 and an application of W. H. Newell entitled "Viewing Field Splitter", Ser. No. 509,587, both of which were filed on even date herewith and are assigned to the same assignee as the present invention.
One of the other features of that system is a mirror array having three reflecting surfaces arranged to form three mutually perpendicular mirrors to permit scanning the mask and wafer together and to result in an exposure equivalent to that of a contact print. This arrangement is also disclosed in the aforementioned application Ser. No. 339,860 and is furthermore disclosed and claimed in an application of D. A. Markle, Ser. No. 509,598, filed on even date herewith and assigned to the assignee as the present invention.
The most obvious method of viewing the wafer and the image thereon would be through the back of the master. However, such viewing suffers from various disadvantages. Among these are the incapability of viewing all of the image surface where the object is opaque, or mainly so, and the incapability of viewing the image plane in one color and the object imaged on the image plane in another color.
In view of these drawbacks, the need for an improved arrangement to permit viewing all image surfaces and to enable viewing the image plane in one color and the object imaged on the image plane in different color is evident.